The present invention relates to graphic arts reproduction, and more particularly to a liquid transfer article for use in transferring an accurately metered quantity of a liquid to another surface and a method for producing it by digital imaging photopolymerization.
One example of a liquid transfer article is a surface on a cylinder, belt, sleeve or plate that is used in printing processes to transfer a specified amount of a liquid coating material, such as ink or other substances, from the liquid transfer article to another surface or substrate. The volumetric capacity of the liquid transfer article is dependent upon the selection of size, shape, and number of cells per unit area. Cells comprise discrete areas on the surface of the liquid transfer article which hold the liquid. Varying these factors permits a high degree of precision in determining print densities. In addition, by controlling the location of the cells on the surface, a precise, predetermined image may be transferred to a receiving surface.
One example of such an article is a gravure surface which includes a pattern of cells or depressions adapted for receiving the liquid coating material. The area of the surface at a common level surrounding the pattern of cells is the land area surface. When the liquid coating material is applied to the article, the cells are filled with the liquid while the remaining land area surface of the article is removed by a wiper or doctor blade. Since the liquid coating material is contained only in the pattern defined by the cells, it is this pattern of liquid that is transferred to the other surface when contacted by the liquid transfer article.
Another example of a liquid transfer article is an anilox surface. The major difference between a gravure surface and an anilox surface is that the entire anilox surface is patterned whereas with a gravure surface only portions are patterned to form a predetermined image. The anilox surface is typically etched with an array of closely spaced, shallow cells or depressions. The liquid coating material flows into the cells as the anilox surface contacts a reservoir. The anilox surface is then scraped with the doctor blade to remove excessive liquid coating material. The remaining liquid coating material in the cells transfers over to another surface when contacted.
In both types of liquid transfer articles, care needs to be taken to ensure that the land area surface is as smooth as possible. If the land area surface of the liquid transfer article is too coarse, removing the excessive liquid coating material from the land area surface of the coarse article becomes problematic resulting in the transfer of too much liquid onto the receiving surface and/or on the wrong location. Therefore, the land area surface of the liquid transfer article should be finished and the cells clearly defined so that they can accept a desired amount of the liquid coating material.
Prior art methods of producing liquid transfer articles having involved either etching a surface of a copper-plated printing element with chemicals or a high energy beam, such as a laser or an electron beam, or photopolymerization of a polymer onto a support base. In the former method, chemical etching is a time-consuming process that involves that use of multiple images in order to prepare a surface for etching. Laser etching, although faster than chemical etching, results in the formation of cells with a new recast surface about each cell and above the original surface or land surface area of the liquid transfer article. The recast surfaces have an appearance of a miniature volcano crater about each cell. This is caused by solidification of the molten material thrown from the surface when struck by the high-energy beam and their formation causes significant problems. As mentioned above, in order to transfer the liquid coating material in a controlled manner determined by the cell pattern, excess liquid has to be completely removed from the liquid transfer surface, for example by a doctor blade. Any excess liquid coating material remaining on the liquid transfer surface after running under the doctor blade will be deposited on the receiving product where it is not wanted and/or in undesired amount. With a laser-etched liquid transfer surface, the doctor blade cannot completely remove excess liquid from the image transfer surface due to the recast surfaces which retain some of the liquid. Thus, it is desirable in most printing applications to have a liquid transfer article which is void of recast surfaces.
Additionally, it has been noted that it is extremely difficult to control the depth and size of all the cells using laser-etching techniques which produce liquid transfer articles having printed patterns. Specifically, the laser is generally required to be activated only where cells are required and inactivated when no cells are required. Unfortunately, the laser start and stop response is not the same response that is achieved once the laser is operating for a set period. For example, when the laser is started, the first few pulses of radiation are less than the energy content of the laser beam for pulses produced after the laser has been operating for a suitable time. This in turn results in the shape and depth of the first few cells in the surface of the article being different from consecutive successive wells formed in the surface of the article.
Consequently, the cells defining the boundary of the pattern are not the same depth and/or size as the cells contained within the center of the pattern and therefore would be incapable of containing a desired volume of liquid. This results in the boundary of the pattern transferred to a receiving surface being off shaded with respect to the overall pattern. In other words, the edges of the printed pattern are somewhat fuzzy. This can result in different shades of the printed pattern being transferred to the receiving surface. Although laser-etching techniques provide an effective means for producing wells or depressions in the surface of liquid transfer articles, the non-uniformity of the few start and stop pulses of the laser can produce an inferior quality liquid transfer article. As such, typical finely engineered, copper-plated, engraved gravure print rollers are extremely expensive.
With the latter method of photopolymerization, typically a printing plate is formed by first placing a negative on a supporting glass plate. An optically transparent release film is then placed on top of the negative which is subsequently coated with a layer of photopolymerizable resin. A backing sheet is then placed on top of the photopolymerizable resin, and the backing sheet is then covered by another glass sheet. Irradiation by actinic light, such as UV light, through the top glass/backing sheet combination forms a solid floor layer of photoresin, which adheres to the backing sheet. The thickness of the floor layer is less than the total thickness of the photoresin. Irradiation through the lower glass plate negative release sheet selectively hardens the photoresin to form an image-printing surface which mirrors the image on the negative. The hardened regions adhere to the floor layer, but not to the transparent release sheet. Subsequent processing removes unhardened (liquid) photoresin to reveal a relief image.
When following the teachings of the prior art, the photopolymerizable resin layer can be placed on the glass plate and a capping blade can be drawn across the resin layer so as to level the layer of resin on the glass plate. The result is a relatively constant thickness resin layer formed on the supporting glass plate in the printing plate production assembly. The uniform layer of resin is then exposed to a UV light source through the negative so as to produce cross-linked solid areas in the resin layer which form a printing image or pattern in the resin layer. The non-cross-linked liquid portions of the resin layer are then removed from the plate, and the result is a selectively relieved cross-linked resin-printing pattern on the plate. The photo negatives required for this type of process can be both costly and time-consuming to produce.
U.S. Pat. No. 5,877,848 to Gillette, et al attempts to overcome the above-mentioned problems by disclosing a method of producing liquid transfer articles by extruding a predetermined thickness layer of a photopolymerizable resin, and then moving the extruded resin layer past a variable intensity light source. The intensity of the light source can be controlled by a preprogrammed microprocessor in several ways. One way of providing the variable intensity light source involves the use of a bank of lights which can be selectively turned “on” and “off”, or can be selectively dimmed or brightened, by the use of microprocessor-controlled switches or rheostats. Selective cross-linking of the resin can be performed within the extrusion die, or the resin can be extruded onto a moving transparent support plate, and the variable intensity light source can be positioned above or below the support plate. In either case, the variable intensity light source may be controlled by a preprogrammed microprocessor, as described above.
Alternatively, the intensity of the light source may be controlled by the use of preprogrammed video signals in conjunction with a suitable video image-producing device. Although, the method disclosed by Gillette, et al is an improvement over previous methods, there still remains a need for faster printing plate production by photopolymerization, as the printing plate of Gillette is formed incrementally by serially cross-linking adjacent section of a layer of cross-linkable resin.
Accordingly, there remains a need in this art for a liquid transfer article which can be accurately imaged without a mask or laser, thereby lowering costs.